Known features of such a dc/dc converter 1 as shown FIG. 1 will be hereinafter described. A direct voltage source 2 supplies through connecting means 3 primary windings 4, 5 of two transformers 6, 7 respectively. In the case represented in FIG. 1, the switching means 3 are in the form of a relay 3. Relay 3 comprises a contact 8 and a control coil 9. Current in the control coil 9 is controlled by a circuit 10. The first transformer 6 is a transformer with a low reluctance which can be assimilated to a current transformer. The second transformer 7 is a voltage transformer. In the case represented in FIG. 1, the dc/dc converter is a medium power, self-oscillating, push-pull converter. The currents that can circulate in the primary winding 4 of first transformer 6 are under control of switching transistors 11, 12 whose collector-to-emitter current is dependent upon the value of polarization current of their respective bases 13 14. The polarizing current of each base 13, 14 of switching transistors 11, 12 respectively is coming from secondary windings 15 of transformer 6 and from a feed-back 52, 53 coming from a secondary winding 23 of second transformer 7. Such a dc/dc transformer works as explained below.
When the magnetic flux in a core 16 of first transformer 6 is increasing the current induced in the secondary 15 is in a first direction, and, for instance, current in the secondary is allows first switching transistor 11 to be on. A current in the primary is then flooding through a first winding 17 of primary winding 4, the collector-emitter of first transistor 11, and the primary winding of second transformer 7. When core 16 of first transformer 6 is saturated, flux in the core is no longer increasing, and consequently no more current is induced in the secondary 15 of this transformer. Transistor 11 is off, closing the circuit made by first winding 17, transistor 11 and primary 5 of second transformer 7. Magnetic field in the core 16 will then decrease, whose decreasing creates a decreasing flux in core 16 and then a current in the secondary 15 of first transformer 6. This current is in the opposite direction as the one which was previously circulating. This current will fire on transistor 12 and a current will circulate through a second winding 18 of primary winding 4. The same phenomenon as the one described above will recur at a frequency which is dependent upon the characteristics of the core 16, the windings 4 and 5 and the polarizing means 49, 50 which are connected between secondary winding 15 of first transformer 6 and bases of transistors 11, 12. Such an oscillating phenomenon needs to be initiated, this is done through a starting circuit 22.
The oscillating phenomenon that has just been described induces square waves in the primary windings 5 of the second transformer 7. This produces a square wave output across the secondary windings 19 of said second transformer 7. Those square waves are rectified through rectifiers 20, integrated, and regulated through regulating and integrating circuit 21, in order to get a desired direct voltage. Generally, in a power supply for a cathode ray tube, such as the ones in television sets or monitors, said direct voltage is fed to a full range switch mode power supply (SMPS) (not shown).
One of the most important design criteria for a power supply is reliability. That means among other features in such converters, having over current protection. This is difficult, due mainly to the fact that the input is connected to a low input voltage source; as a consequence, for medium power, the current flowing through transistors 11 or 12 may be high, for instance more than 10 amperes.